1. Field of the Invention
The present invention relates to a Ni--Cr--Fe based alloy steel of high corrosion resistance against hot molten salts containing chlorides and/or alkali oxides. Particularly, the present invention relates to a Ni--Cr--Fe based alloy steel which has a low Cr content so that its corrosion resistance to hot molten salts can be greatly improved. Also, the present invention is concerned with the use of the alloy steel in the structural materials and the structural components for treating hot molten salts.
2. Description of the Prior Art
With characteristic physicochemical properties, such as high electrical conductivity, high concentrated and convenient treatment and flowability, molten salts have been utilized in various industrial techniques, especially in jet engines, fuel cells, and catalysts. In addition, molten salts are applied for solar energy utilization and metal refining. Accordingly, active research has also continued to be directed to methods and vessels for treating molten salts, particularly at high temperatures and to corrosion-resistant materials for the vessels.
For instance, sulfate-based molten salts, exemplified by Na.sub.2 SO.sub.4, Na.sub.2 SO.sub.4 --NaCl, Na.sub.2 SO.sub.4 --V.sub.2 O.sub.5 and Na.sub.2 SO.sub.4 --Li.sub.3 SO.sub.4 are usually used in jet engines and gas turbines. As materials for this type of equipment and for use in electrochemical test for corrosion resistance to such sulfate-based molten salts, there are known Inconel 600, Inconel 825, Nimocast 713, SUS 304, SUS 310, MA 956, SS41 and Incoloy 800 (Wu, C. X., Corrosion control-7th APCCC, Vol. 1, pp 136-141, 1991; Santorelli, R., Mater. Sci. Eng. A120-A121, (1-2), 283-291, 1989).
Carbonate-based molten salts, exemplified by Li.sub.2 CO.sub.3, Na.sub.2 CO.sub.3 --NaCl, Na.sub.2 CO.sub.3 --Na.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 --K.sub.2 CO.sub.3, can be found in fuel cells, reactors and boilers. These vessels are usually made of Inconel 600, X2 (16.5-18.5% Cr, 11-14% Ni, 2-2.5% Mo), X12 (24-26% Cr, 19-22% Ni) or an alloy steel comprising 30% Cr-45% Ni-1% Al-0.03% Y (Park, H-H., J. Society Material Engineering for Resources of Japan, 10(2), 18-26, 1997; Sasaki, M., Corrosion Engineering, 45(4), 192-200, 1996).
As for nitrate-based molten salts, exemplified by NaNO.sub.3, Ba(NO.sub.3).sub.2, NaNO.sub.3 --KNO.sub.3, etc., they are used for heat recovery. SUS 304, SS 41, Inconel 600 (Inco Alloys International, U.S.A.), Inconel 625, Hastelloy-N, and Hastelloy-X are electrochemically tested for corrosion resistance to such molten salts and used for the structural component treating the said molten salts (Ebara, R., J. Jpn. Inst. Met., 52(5), 508-516, 1988; Nishikata, A., J. Jpn. Inst. Met., 45(6), 610-613, 1981).
It is also known that SUS 304 and Hastelloy-N are resistant to the corrosion caused by halide-based molten salts, exemplified by LiCl--KCl, LiF--KF, LiF--NaF--KF, KCl--BaCl.sub.2 --NaF, KCl--NaCl--NaF, etc. (Iwamoto, N., Trans. JWRI., 9(2), 117-119, 1980).
Other references directed to alloy steels with corrosion resistance to molten salts are found in many patents.
Japanese Pat. Laid-Open Publication No. Heisei 8-41595 discloses an Fe--Ni--Cr based alloy steel used where there is needed high corrosion resistance to chloride-based molten salts, specifying a composition which comprises 0.05-1.5% of Mn, 18-30% of Cr and 10-35% of Ni under the condition of Cr/Fe=0.33-0.7 and Ni/Fe=0.33-1.0. Japanese Pat. Laid-Open Publication No. Heisei 5-279811 suggests an alloy steel composed mainly of 2% or less of Si, 1% or less of Mn, 25-40% of Co, 12-18% of Cr, 10-40% of Ni, 2-4% of Mo, and 8% of W as a material for boilers superior in corrosion resistance to molten salts.
U.S. Pat. No. 5,223,214 describes an alloy steel used where resistance to heat and corrosion is needed as in, for example, boilers and waste incinerators, which is composed mainly of 10.5-28% of Ni, 14.8-23% of Cr, 3-6.6% of Si, 0-4% of Al, 0.15-1.6% of Mo, and 0.25-1.25% of W. Canadian Pat. No. 2,084,912 introduces a corrosion-resistant alloy steel for boilers, which comprises 10-25% of Co, 18-28% of Cr, 10-50% of Ni, 2-4% of Mo, and 8% or less of W.
Japanese Pat. Laid-Open Publication No. Heisei 7-268565 is directed to an alloy steel which is composed mainly of 2-4% of Si, 22-25% of Ni, 24-30% of Cr, and 1-2% of Mo. It is described that the alloy steel is superior in hot workability and shows high corrosion resistance even in a hot condition comprising hydrochloride gas, molten salts, sulfuric acid, and/or alkali, so that it is useful as a material for steam boilers which are usually operated at high temperature and high pressure. Japanese Pat. Laid-Open Publication No. Heisei 5-117816 discloses an alloy steel useful for heat exchangers and heat engines, which are usually exposed to hot, corrosive environments comprising sulfates and chloride-based molten salts. The alloy steel comprises mainly 12-30% of Ni, 18-30% of Cr and 2% or more of Mo.
In Japanese Pat. Laid-Open Publication No. Sho. 57-39159 is disclosed an Al.sub.2 O.sub.3 -coated austenite alloy steel resistant to oxidation and heat, comprising 10-40% of Ni, 11-32% of Cr, 4.5-9% of Al, 3% or less of Si and 2% or less of Mn. Composed of 5% or less of Si, 1.5% or less of Mn, 8-70% of Ni, and 15-35% of Cr, an alloy steel resistant to molten borax-caused corrosion-resistant alloy steel is disclosed in Japanese Pat. Laid-Open Publication No. Sho. 56-150162. An alloy steel disclosed in Japanese Pat. Laid-Open Publication No. Sho. 190143 is used in molten carbonate type fuel cells, comprising 1% or less of Si, 2% or less of Mn, 15-35% of Ni and 15-35% of Cr.
Japanese Pat. Laid-Open Publication No. Heisei 6-145857 discloses an alloy steel for boilers, which is highly resistant to molten salt-caused corrosion in addition to being good in on-site workability. The alloy steel is prepared from a composition comprising 2.5% or less of Si, 1% or less of Mn, 40-55% of Co, 7-12% of Cr, 10-30% of Ni, 2-4% of Mo, and 8% or less of W. Another alloy steel suitable for boilers is found in Japanese Pat. Laid-Open Publication No. Heisei 5-279785. Showing high corrosion resistance to molten salts, especially molten chlorides, the alloy steel is composed mainly of 2.5% or less of Si, 1% or less of Mn, 40-55% of Co, 7-12% of Cr, 10-30% of Ni, 2-4% of Mo, and 8% or less of W. These last two above-cited references teach that an improvement in corrosion resistance of the alloy steels can be obtained when adding Co, Ni, Cr and Mo in cooperation, but not when alone.
The above-illustrated alloy steels are characterized in that they have high contents of Cr or comprise W, V and/or Mo. Such conventional alloy steels, however, are reported in many articles to be unsuitable as materials for treating single or complex molten salts, including alkali oxides (J. A. Goebel, F. S. Pettit and G. W. Goward, Met. Trans. 4, 261 (1973)). Particularly, chloride-based molten salts are so highly hydrophilic that they are easily hydrated when being exposed to the air. Thus, changes occur in the composition of the molten salts, having great influence on the corrosion resistance of the alloy steels. Coexistence of molten salts and oxides makes their physical and chemical properties more complex, resulting in accelerating corrosion. Thus far, sufficient research has not been done in regard to the complex corrosive situation. Further, the conventional alloy steels cannot sustain themselves for a long period of time in such molten salt's corroding conditions. Particularly, no alloy steels have been developed which are of high corrosion resistance to molten salts containing alkali oxides such as LiCl--Li.sub.2 O.
Before the present invention is disclosed or described, it must be noted that, as used in the specification and the appended claims, the term "%" means weight % unless the context clearly dictates otherwise.